Currently, there is a growing demand for high power ultraviolet (UV) pulsed lasers for various industrial applications such as LED scribing, chip dicing, via-hole drilling, plastics marking and others. In comparison with more common IR lasers, UV lasers have an advantage of higher linear and nonlinear absorption of the UV light by some materials and their possibility to achieve smaller focus spots. The majority of commercially available pulsed UV lasers are diode pumped solid state Nd:YVO4, Nd:YAG, or Nd:YLF lasers or Yb-doped fiber master oscillator power amplifier (MOPA) lasers operating near 1 μm wavelength with inter- or intra-cavity frequency tripling or quadrupling.
The conventional way of third harmonic generation (THG) employed in most of the UV lasers operating near 0.35 μm consists of a two stage process: second harmonic generation (SHG) in a type-I phase-matched nonlinear optical crystal and sum frequency generation of the fundamental and second harmonics in a type-II phase-matched crystal. Usually LiB3O5 (LBO) crystals are used for both processes due to their high damage threshold, high nonlinearity, low absorption in visible and UV ranges, and high crystal growth yield. The popularity of the described scheme can be explained by its ease of implementation: in the output of the first nonlinear crystal the fundamental and double frequency waves are polarized in the orthogonal planes which is exactly what is required for type-II phasematching condition in the second nonlinear crystal. Because of that there is no wave manipulation needed between the nonlinear crystals except for focusing of the waves.
The alternative way of THG is to use type-I phase-matched crystals for both processes. Compared to the above-discussed technique, this one is associated with higher conversion efficiency of sum-frequency generation under type-I phasematching condition. For example, the total conversion efficiency is about 2.2 times higher in the type-I phase-matched LBO crystal than the one in the type-II crystal at 355 nm wavelength at 100° C. In addition, there is no spatial walk-off of the fundamental and second harmonic waves in the type-I phase matching scheme, which removes the crystal length limitation present in type-II phasematching. There is, however, non-zero spatial walk-off of the third harmonic wave with respect to the fundamental and the second harmonic waves. This effect leads to some ellipticity of the 355 nm output wave, which is considered to be a minor problem and could be compensated, for example, by an anamorphous prism arrangement. Thus due to higher nonlinearity and the absence of crystal length limitation, THG in a type-I LBO crystal is significantly more efficient. This is especially important for devices with low IR pump peak powers, such as fiber lasers. Higher efficiency also decreases a rate of crystal degradation by relaxing the focusing conditions in the THG crystal.
The peculiarity of the type-I phase-matching scheme, however, is the fact that both fundamental and double frequency waves must be polarized in the same plane. That means that a polarization control element is required after the first nonlinear crystal. Usually a birefringent phase plate is inserted between the nonlinear crystals for that purpose. This birefringent phase plate should simultaneously provide a half wavelength phase shift to the fundamental wave and a whole wavelength phase shift to the second harmonic wave. If the phase axis of such a birefringent phase plate is oriented at 45° with respect to the polarization plane of the fundamental wave, the birefringent phase plate will flip the fundamental wave polarization at 90° degrees, while the polarization of the second harmonic wave will remain unchanged. As a result, the polarizations vectors of the fundamental and second harmonic waves become parallel.
As one of ordinary skill in the geometric optics is well aware, the use of the birefringent phase plate may be problematic. One of the known problems includes the dependence of the phase shift from temperatures fluctuations which typically leads to the third harmonics power instability. Another problem sterns from resonant wavelength dependence that requires very high precision manufacturing of the phase plate, which, in turn, drives the cost up. Still another problem relates to the requirement of precise angular adjustment around the wave propagation axis relative to the polarization plane which complicates the birefringent phase plate installation.
As also known to one of ordinary skill, because of the dispersion of the difference of the ordinary and extraordinary indexes of refraction |no−ne|, it is impossible to make a birefringent phase plate which ideally works at both fundamental and second harmonic wavelengths. As a result the required birefringent phase plate will have a considerable thickness and therefore it will have a higher thermal dependence. For example, in order to get an acceptable angular mismatch between the fundamental and second harmonic waves polarization planes of Δδ≈0.3° one has to use a 91th order 5.55 mm thick quartz phase plate. A simple calculation shows that the temperature change from 0 to 50° C., which is a typical industrial applications operation temperature range, leads to a prohibitively high change of the polarization planes angular mismatch of Δδ=110°.
Based on the foregoing, a need therefore exists for a frequency converter configured with an optical active crystal avoiding problems associated with the above-discussed birefringent phase plate.
A further need exists for a method of sum-frequency conversion utilizing the disclosed optical active crystal which is carrying out a role of a thermo-stable non-resonance rotator of polarization planes.